Hot loop oscillations seen by SUMER: examples and statistics

Physics

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Solar Flares, Coronal Oscillations, Uv Radiation, X-Rays

Scientific paper

We measure physical parameters of Doppler-shift oscillations in 17 flare-like events. These events have been recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO, along a slit fixed above limb active regions. The selected spectral windows contain emission lines with formation temperatures from ~104 to 107K. The events were only detected in hot flare lines, without any signature in lines formed around 2×106K. Similarly, the Doppler shift oscillations occur in regions coincident with hot soft X-ray loops, but not with EUV loops. The oscillations have periods of 11 - 31 min, with an exponential decay time of 5.5 - 29 min, and show an initial large shift pulse with peak velocities up to 200 km s-1. Several indications suggest that the Doppler oscillations are incompressible coronal loop oscillations, that are usually excited impulsively by weak flare (or microflare) events that also produced a strong emission increase at 5 - 8×106K.

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